1. Field of the Invention
The present invention relates to an optical coupling structure, which will be called simply "optical coupler" hereinafter, used in an optical communication apparatus or an optical information processor. In particular, the present invention relates to an optical coupler for optically coupling a light beam emitting semiconductor device, such as a semiconductor laser, with a single-mode optical waveguide or a single-mode optical fiber.
2. Description of the Related Art
In order to optically couple two pieces of single-mode type optical components to each other at high efficiency, it is required to perform mode matching and optical axis alignment between the two optical components. Herein, the mode matching is to make optical intensity distributions (which is also called "mode size") in the single-mode type optical components equal to each other, and the optical axis alignment is to make optical ales of the optical components coincide with each other. FIGS. 1, 2A, 2B, 3, 4, 5, 6, 7A, 8A, 9A, and 10A are schematic diagrams for showing several methods of the axis alignment in the prior art.
FIG. 1 is a schematic diagram of an optical coupler of the prior art in a case where a light emitting semiconductor device 200 like a semiconductor laser is optically coupled to an optical fiber 3 by using first and second lenses 41 and 42. There will be cases hereinafter where the light emitting semiconductor device 200 is called "semiconductor laser 200" or simply "laser 200". In FIG. 1, the laser 200 and the optical fiber 3 are placed on a center line, or axis, 4 so as to be opposite to and aligned with each other. The first lens 41 gathers light emitted from the laser 200, producing a light beam almost in parallel with the center line 4, and the second lens 42 converges the light beam on the fiber 3, giving a proper size. For example, at an entrance of the fiber 3, a spot size (1.2 .mu.m.times.1.7 .mu.m.about.2.1 .mu.m.times.3.2 .mu.m) of the light beam is magnified 4.about.6 times by the lens 42, so as to be matched with a mode size (approximately 10 .mu.m) of the fiber 3. As described above, by virtue of the first and second lenses 41 and 42, the mode matching can be performed between the semiconductor laser 200 and the optical fiber 3 at high coupling efficiency.
FIGS. 2A and 2B respectively show a case where a laser 200 is hermetically sealed in a housing, or box, 600, for preventing the laser 200 from being damaged due to moisture and oxidation. In FIGS. 2A and 2B, the same reference numerals as used in FIG. 1 designate the same parts as in FIG. 1. In FIGS. 2A and 2B, the laser 200 is mounted on a sub-mount 603 placed on a mounting block 601, and the laser 200, the sub-mount 603 and the mounting block 601 are installed in the airtight box 600 having an optical window 607. Light emitted from the laser 200 is gathered to form a light beam by the first lens 41 and the light beam arrives at the second lens 42, passing through the optical window 607. The light beam is converged on the optical fiber 3 by the second lens 42, so as to have a size proper to the mode matching. The optical fiber 3 is inserted into a fiber coupling unit 32, supported by a ferrule 31.
FIG. 3 is another optical coupler using a tapered optical fiber having a spherical tip at an end thereof opposite to a semiconductor laser. In FIG. 3, the same reference numerals as in FIG. 1 designate the same device as in FIG. 1. In FIG. 3, an optical fiber 3A is tapered near its end and a tip of the tapered end has small lens structure. When the optical fiber 3A is placed so that the tip is optically coupled to a light emitting active layer 201, which will be simply called "active layer 201" hereinafter, of the laser 200, the mode size of the light beam emitted from the active layer 201 is decreased by the small lens structure so as to coincide with the mode size of` the optical fiber 3A. This results in increasing the coupling efficiency between the laser 200 and the optical fiber 3A.
FIG. 4 shows a case where the laser 200 is sealed in an air-tight box, or housing, 600. In FIG. 4, the same numerals as in FIGS. 1 and 2A designate the same parts as in FIGS. 1 and 2A. In FIG. 4, the surface of the optical fiber 3A is metal-coated (not depicted in FIG. 4) and the optical fiber 3A is fixed to the mounting block 601 with solder 611, in the airtight box 600.
Hereupon, in case of the optical couplers shown in FIGS. 1, 2A and 2B, the position adjustments are precisely performed throughout the laser 200, the lenses 41 and 42 and the optical fiber 3. In case of the optical couplers shown in FIGS. 3 and 4, the position adjustment is precisely performed between the laser 200 and the optical fiber 3A. The position adjustments are performed in FIGS. 1, 2A, 2B, 3 and 4 by actually operating the laser 200 and measuring intensity of the laser light beam, passing through the optical couplers, while carefully changing the relative positions of the semiconductor laser, the lenses and the optical fiber, until the measured light intensity becomes maximum.
In order to avoid performing the careful position adjustment, the position adjustment has been improved in the prior art by several ways as shown in FIGS. 5, 6, 7B, 8A, 9A and 10A.
FIG. 5 is a schematic perspective view for illustrating a typical example of an improved position adjustment in the prior art. In FIG. 5, the same reference numerals as in FIG. 1 designate the same parts as in FIG. 1. In FIG. 5, a V-shaped groove 181, bonding pads and positioning mars are formed on a silicon substrate 180. The bonding pads and the positioning marks on the silicon substrate are not depicted in FIG. 5, but are depicted in FIGS. 11A and 11B with reference numbers 102 and 107 respectively. The V-shaped groove 181 is fabricated on the surface of the silicon substrate 180 in a photo-lithography process, for guiding the optical fiber 3. By virtue of applying the V-shaped groove 181 to an optical coupler 1B, the position adjustment of the optical fiber 3 can be performed smoothly.
Meanwhile, in order to position the laser 200 on the silicon substrate 180, other bonding pads and positioning marks are formed on the laser 200 so as to be connected with the bonding pads 102 and to be corresponded to the positioning marks 107 on the silicon substrate 180 respectively. The positioning marks and the bonding pads on the laser 200 are not depicted in FIG. 5; however, they are similar to the positioning marks 204 and bonding pads 202 shown in FIGS. 11A and 11B. The position adjustment of the laser 200 is carried out by using all alignment apparatus (not depicted in FIG. 5), using a groove 182 formed oil the silicon substrate 180. The alignment apparatus moves the laser 200 so that the positioning marks on the laser 200 coincide precisely with the positioning marks on the silicon substrate 180. After the position adjustment is over, the laser 200 is fixed to the silicon substrate 180 by bonding. The above position adjustment is detailed in IEEE TRANSACTIONS ON COMPONENT, HYBRIDS, AND MANUFACTURING TECHNOLOGY, VOL. 15, NO. 6, pp.944-955 (1992). The optical coupler having the V-shaped groove, positioning marks and bonding pads as shown in FIG. 5 will be numbered "1B" hereinafter.
FIG. 6 shows another optical coupler of the prior art, which is similar to the optical coupler in FIG. 5 except that the optical fiber is a tapered optical fiber 3A. The optical coupler is numbered as "1C" in FIG. 6.
FIGS. 7A and 7B show another optical coupler of the prior art in a case where a semiconductor laser is in an air-tight sealed enclosure. In FIGS. 7A and 7B, the same reference numeral as in FIG. 5 designates the same component or part as in FIG. 5. FIG. 7A shows the optical coupler 1B before being sealed air-tight, and FIG. 7B shows the optical coupler 1B after being sealed air-tight air-tightened].
The optical coupler 1B is sealed air-tight by: placing the optical coupler 1B in a space formed by a protruded frame formed on a substrate 700, providing a notch at a side of the frame 702 for passing the optical fiber 3 therethrough, as shown in FIG. 7A; and applying an epoxy base adhesive on and around the frame 702 and putting a cover 400 on the substrate 700, and air-tight sealing the optical coupler 1B by filling up a gap between the cover 400 and the substrate 700, as shown in FIG. 7B. When the epoxy base adhesive such as a plastic material is used, the air-tight sealing is hard to be realized in the strict sense of the word. However, "air-tight sealing" is used hereinafter in a sense that the laser component can be protected from corrosion due to the moisture or a corrosive gas by the sealing performed by the epoxy base adhesive.
FIGS. 8A and 8B show another optical coupler of the prior art, including a multimode optical waveguide. FIG. 8A is a schematic perspective view of the optical coupler and FIG. 8B is an elevation view of the optical coupler, looked from the left-down side of the optical coupler in FIG. 8A. In FIGS. 8A and 8B, the same reference numerals as in FIG. 1 designate the same parts as in FIG. 1. In FIGS. 8A and 8B, an optical waveguide 300 and four projected objects 300A, which will be called "a stand-off 300A" hereinafter, are formed on a substrate in a two layer structure consisting of a cladding layer and a core for forming the optical waveguide 300. That is, the optical waveguide 300 and the stand-off 300A are formed on the substrate 100 at the same time. The optical waveguide 300 is a multimode waveguide, so that the thickness of the cladding layer 301 is more than 50 .mu.m. The stand-off 300A is used for settling and positioning the laser 200 on the substrate 100. In order to settle the laser 200 onto the stand-off 300A, a cut off portion is formed at every corner on the down side of the laser 200. The optical waveguide 300 and the stand-off 300A are formed by IC fabrication technology, so that they can be placed precisely in correct position on the substrate 100. Therefore, when the laser 200 is settled to the stand-off 300A, the optical coupling between the optical waveguide 300 and the laser 200 could be performed in high coupling efficiency. However, there is a problem that the laser 200 is hard to be settled in a correct position because the cut off portions 205 are hard to be formed precisely.
FIGS. 9A and 9B show another optical coupler of the prior art, including a different type of single mode optical waveguide 300. FIG. 9A is a schematic perspective view of the optical coupler and FIG. 9B is an elevation view of the optical coupler, looked from the left-down side of the optical coupler in FIG. 9A. In FIGS. 9A and 9B, the same reference numerals as in FIG. 8A designate the same parts as in FIG. 8A. In FIGS. 9A and 9B, the substrate 100 is made of silicon, and the cladding layer 301 and the core 302 are made of quartz. When a single mode optical waveguide is used in the optical coupler as shown in FIGS. 9A and 9B, high optical coupling efficiency can be obtained when the following ratio, called "a relative refractive index difference", is satisfied between the refractive index (n.sub.1) of the cladding layer 301 and the refractive index (n2) of the core 302: EQU {(n.sub.2 -n.sub.1)/n1}.times.100=0.3%.about.0.75%,
and the size of the core is within 5.times.5 .mu.m.about.8.times.8 .mu.m. In FIGS. 9A and 9B, the cladding layer 301 and the core 302 are formed on the silicon substrate 100 so that a thickness (t) of the cladding layer 301 placed between an upper surface of the silicon substrate 100 and a bottom surface of the core 302 becomes more than 30 .mu.m.
FIGS. 10A and 10B show an optical coupler including the single mode optical waveguide 300, which is the same as in FIGS. 9A and 9B except that a spacer is inserted between the silicon substrate 100 and the laser 200. The spacer 5 is used to raise an optical axis of the laser 200 .mu.p for making the optical axis of the laser 200 coincide with an optical axis of the optical waveguide 300 consisting of the cladding layer 301 and the core 302.
As described with reference to FIGS. 1 to 10 (A and B), many methods and apparatus have been used for the optical couplers in the prior art, for obtaining good optical coupling between the optical components. However, there have been the following problems in the position adjustment: the alignment apparatus must be used for precisely adjusting the positions of the lens, the semiconductor laser and the optical fiber; the V-shaped groove must be formed on the substrate for firmly positioning the optical fiber; the positioning marks must be formed on the semiconductor laser and the substrate for correctly positioning the semiconductor laser; the stand-off must be formed on the substrate for positioning the semiconductor laser correctly; and the spacer must be formed on the substrate, for achieving the correct coupling between the semiconductor laser and the optical waveguide.
However, besides the above problems, there have been other problems as described below.
In case of the optical couplers shown in FIGS. 1 to 4, the following problems will occur: it takes a lot of time to perform the position adjustment because the position adjustment must be performed individually between the semiconductor laser and the lens, the lens and the optical fiber, and the semiconductor laser and the optical fiber; and the semiconductor laser is apt to be deteriorated or damaged by a faulty operation which could happen during the position adjustment.
In case of the optical couplers shown in FIGS. 5 to 7A and 7B, there is a merit that the semiconductor laser is not required to be operated during the position adjustment, however, there has been a problem that the mechanical position adjustment is not easy to be performed between the semiconductor laser and the optical fiber. In an optical coupler generally, there is a tolerance, called "coupling tolerance", for keeping the coupling efficiency higher than a designated value, such as 80%, of the maximum coupling efficiency. When an optical fiber has a lens structure, as in the case of the optical fiber 3A in FIG. 6, the coupling tolerance is required to be less than 0.5 .mu.m, and when a tip of an optical fiber is flat as in the case of the optical fiber 3 in FIG. 5, the coupling tolerance is required to be less than 1 .mu.m. From the above, it can be said that the position adjustment is not easily performed mechanically. In particular, when the laser 200 is bonded on the substrate 180 as shown in FIGS. 5 and 6, it is not easy to perform the optical coupling between the optical fiber 3 (or 3A) and the laser 200 with a desirable coupling tolerance. In case of FIG. 5, there has been another problem that the mode mismatching increases between the laser 200 and the optical fiber 3, resulting in increasing the coupling loss as much as 7 dB at least.
In case of the optical coupler shown in FIGS. 8A and 8B, there has been a problem that the cut off portions 205 are not easy to be formed with high accuracy, which produces a problem of decreasing the coupling efficiency.
In case of the optical coupler shown in FIGS. 9A and 9B, there has been a problem as described below.
When the optical waveguide 300 is applied to the optical coupler as shown in FIGS. 9A and 9B, and when high refractive index material such as silicon is used as the substrate 100, the cladding layer 301 formed under the core 302 is required to have a proper thickness for decreasing light arriving at the substrate 100 from the core 302. For example, when the relative refractive index difference is 0.3%.about.0.75% and the size of the core 302 is 5 .mu.m.times.5 .mu.m-8 .mu.m.times.8 .mu.m, the thickness of the cladding layer 301 under the core 302 must be more than 30 .mu.m, as described with reference to FIG. 9B. This criterion is adopted also to the optical waveguide 300 in FIGS. 8A and 8B. However, in case of coupling the optical waveguide with the semiconductor laser as shown in FIGS. 8A and 8B or 9A and 9B, there is a problem that it is not easy to make the height of the optical axis of the optical waveguide coincide with the height of the semiconductor laser.
In order to explain the above difficulty, a bonding method applied to the semiconductor laser will be described with reference to FIGS. 11A and 11B, and the constituents and the dimensions of the substrate and the semiconductor laser flip-chip bonded on the substrate through the bonding pads and the height of the optical axis of the semiconductor laser will be discussed concretely with reference to FIGS. 12A and 12B.
FIG. 11A is a schematic perspective view for illustrating that the laser 200 in FIG. 9A is ready to be mounted on a substrate, and FIG. 11B is a schematic elevation view of the laser 200 mounted on the substrate 100. As shown in FIG. 11A, the bonding pads 202 are formed on the underside of the laser 200 as electrodes and a solder bump 203 for bonding is coated on each of the bonding pads 202. The solder bump 203 is formed of a metal having a low melting point such as solder. The laser 200 is bonded to the substrate 100, by setting the bonding pads 202 on the bonding pads 102 formed on the substrate 100 as electrodes with the insertion of solder bumps 203 therebetween. Then, the laser 200 is bonded to the substrate 100 as shown in FIG. 11B, through the process of heating and cooling the bonding pads 202 and 102 with the solder bump 203.
FIGS. 12A and 12B illustrate minute layer structure of the laser 200 and the silicon substrate 100 described with reference to FIGS. 11A and 11B, respectively. In FIGS. 12A and 12B, the same reference numerals as in FIGS. 11 and 11B designate the same parts as in FIGS. 11A and 11B. In FIGS. 12A and 12B, a wiring pattern connected with the bonding pads 102 is 0.3 .mu.m, an insulation layer is 0.3 .mu.m, the bonding pad 102 is 0.35 .mu.m, the solder bump 203 is 2.about.6 .mu.m, the bonding pad 202 is 0.5 .mu.m, an insulation film 26 is 0.3 .mu.m, a cladding layer 231 is 1.5 .mu.m, and the active layer 201 is 0.14 .mu.m, respectively, in thickness. In the above layer structure, when the laser 200 shown in FIG. 12A is bonded with the silicon substrate 100 shown in FIG. 12B, the height from the upper surface of the silicon dioxide film 101, which will be called "silicon dioxide film or layer 101" hereinafter, on the silicon substrate 100 to the center of the active layer 201 is 5.32.about.9.32 .mu.m. The above height is nothing but a height of the optical axis of the laser 200, which teaches that the height of the optical axis is too low in comparison with the height of the center of the core 302 in FIG. 9A or 9B. In other words, when the laser 200 and the optical waveguide 300 are directly mounted on the substrate 100 as shown in FIG. 9A, a problem occurs that it is very hard to make the optical axes of the laser 200 coincide with the core 302 of the optical waveguide 300.
In order to make the height of the optical axis of the laser 200 coincide with the height of the core 302, the spacer 5 is inserted between the laser 200 and the substrate 100 as shown in FIG. 10A. However, the insertion of the spacer 5 may produce a positional deviation between the optical axes of the laser 200 and the core 302 because of fabrication errors in the spacer 5 and the optical waveguide 300 having the core 302. The amount of the fabrication error depends on the height of the optical axis of the core 302 and the thickness of the spacer 5. For example, if the respective fabrication errors in the core 302 and the spacer 5 are .+-.5%, and when the height of the optical axis of the core 302 is 30 .mu.m, the respective fabrication errors become .+-.1.5 .mu.m, which results in producing a total positional deviation of .+-.3 .mu.m between the laser 200 and the core 302. This positional deviation of .+-.3 .mu.m is too large for the required optical coupling. The above explanation is as an example; however, these amounts of the fabrication errors are frequently produced, so that there is also a problem that it is hard to obtain required coupling efficiency when the spacer is inserted between the semiconductor laser and the substrate.